This invention relates to a noise shaping circuit for reducing the quantization error produced in a quantizer for audio signals.
Heretofore, when sampling input signals at a sampling frequency fs, quantizing the sampled input signals into digital signals and requantizing the digital signals for reducing the number of bits, quantization noises produced at the re-quantizer are frequently fed back via a noise filter to an input side of the quantizer to reduce the quantization noise or quantization distortion by way of performing a quantization error reducing operation by error feedback referred to hereinafter as noise shaping. It is the noise shaping circuit which carries out the noise shaping. With the noise shaping circuit, the spectrum of the quantization noise is moved to a frequency range outside the audible range, for example, to a higher frequency range outside the audible range, to improve the S/N ratio within the audible range.
In general, in a first order noise shaping circuit of a basic construction, the quantization noise is found from a difference between an input and an output of a quantization circuit. However, when carrying out this operation of finding the difference, a large number of adders and flip-flops are required in a practical system for inputting data of several bits, such that the adders and the flip-flops account for about 30% of the first order noise shaping circuit, thus enlarging the circuit scale.